1. Field of the Invention
The present invention relates generally to the fabrication of electric microswitches with thin semiconductor films and column-row (x-y) addressable electric switch matrices constructed with such microswitches. These microswitches are two terminal devices through which electric current, electric potential or their derivatives or integrals can be switched on and off by the magnitude or the polarity of an external bias. They are made of semiconducting thin films in metal/semiconductor/metal, thin film configuration. Column-row addressable electric micro-switch matrices can be made to cover large areas, with high pixel density. Such matrices can be integrated with one (or several) additional layer(s) with electronic properties which vary in response to external physical conditions (such as photon radiation, temperature, pressure, x-rays, magnetic field and so on), thereby forming a variety of detector matrices.
2. Brief Description of the Prior Art
Traditional electric switches are electromechanical devices such as relays for large current, high power applications. On the other hand, there is general interest in high pixel density, column-row addressable electric microswitches for various sensor applications. Switches made with discrete mechanical relays are too bulky, too large in size and often too slow in switching speed for this application. Fewer than 102 channels are typically seen in a single control board in the automation control industry.
Complementary metal-oxide-semiconductor (CMOS) technology and field effect transistors have been used to fabricate switching circuits in large scale integrated circuits (LSICs) on semiconductor wafers. Typicall switching circuits are constructed by a series of field effect transistors and are known as active matrix arrays. Such microswitches have been used in fabrication of high pixel density 2D image sensors and memory devices. However, the material and the process costs have limited the use of such active matrix arrays in large size sensor applications.
The thin film transistor (TFT) technology on glass or quartz substrates, developed originally for the needs of liquid crystal displays (LCDs), provide another example of active-mode (AM) microswitch substrates. In addition to use in AM-LCDs, a large size, full color image sensor made with amorphous silicon (a-Si) p-i-n photocells on a-Si TFT panels was demonstrated recently [J. Yorkston et al., Mat. Res. Soc. Sym. Proc. 116, 258 (1992); R. A. Street, Bulletin of Materials Research Society 11(17), 20 (1992); L. E. Antonuk and R. A. Street, U.S. Pat. No. 5,262,649 (1993); R. A. Street, U.S. Pat. No. 5,164,809 (1992)].
FETs are three-terminal, active devices. Microswitch panels made with such switch units are often called active matrices. The drain current of each FET can be switched on and off by its gate voltage. The on/off ratio is typically in the range of 104-108.
As demonstrated in this invention, solid state microswitches can also be made from two-terminal, passive devices such as metal/semiconductor Schottky diodes, metal/semiconductor/metal (MSM) devices, p-type semiconductor/n-type semiconductor (p-n) junction devices, or p-type semiconductor/insulator or undoped semiconductor/n-type semiconductor (p-i-n) junction devices. The electric current can be switched on and off by the magnitude or the polarity of an external bias.
This invention discloses large size, high pixel density microswitch matrices comprising passive devices in MSM structure or in structures of its variations. This invention also discloses a method of fabricating such large size, high pixel density microswitch matrices. The semiconductor, which can be either organic or inorganic, is in a thin film configuration. Thin film devices made with inorganic materials (such as selenium, germanium, silicon, Ge—Si alloys, ZuS, CdS or CdSe) have been developed for decades, and have been used in many applications, including for example photovoltaic energy conversion. Organic diodes in the metal-organic-metal MSM thin film structure have also been studied [for reviews of MSM devices made with organic molecules and conjugated polymers, see: James C. W. Chien, Polyacetylene: Chemistry, Physics and Material Science, Chapter 12 (Academic, Orlando, 1978); G. A. Chamberlain, Solar Cells 8, 47 (1983); J. Kanicki, in Handbook of Conducting Polymers, T. A. Skotheim, Ed. (Dekker, N.Y., 1986)]. However, the performance of these early devices (as determined by their I-V characteristics) was insufficient to enable use as electric switches.
With the improvement of both material quality and device fabrication processes, organic MSM devices with rectification ratios of 105-106 were recently demonstrated [D. Braun and A. J. Heeger, Appl. Phys. Letters 58, 1982 (1991); G. Yu, C. Zhang and A. J. Heeger, Appl. Phys. Lett. 64, 1540 (1994)].
The rectification ratio can be further improved by introducing proper blending processes and by structural variation of the device, such as using a bi-layer semiconducting film or by selecting different metals as contacts to improve the carrier injection [I. Parker, J. Appl. Phys. 75, 1656 (1994)]. Such organic MSM devices can be operated continuously over periods in excess of 104 hours at current densities of 10 mA/cm2 [G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Annual Conference of Materials Research Society, San Francisco, April 1995].
As disclosed in this invention, these thin film MSM devices with high recitification ratio can be used for fabricating large area solid state microswitch boards (panels) with high pixel density.